Sputter target made of a silicon alloy and process for producing a sputter target

ABSTRACT

A sputter target produced from an alloy powder, the individual particles of which are composed of an Si—Al alloy. It is found that a high degree of homogeneity of the sputter target can be achieved in this way.

BACKGROUND OF THE INVENTION

[0001] The invention concerns a sputter target that is composed essentially of silicon and aluminum and a process for producing this type of sputter target.

[0002] Sputter targets made of an Si—Al alloy are often used in thin-film technology for the reactive sputtering of optically functional coatings of Si₃N₄ or SiO₂. In principle, it is the optical properties of SiO₂ and Si₃N₄ that are of interest, for the aluminum is usually added only to produce sufficient electrical conductivity of the sputter target or to reduce its brittleness and susceptibility to cracking. With respect to the desired function of the thin film that is to be produced with the use of the target, the aluminum content of the target is useless and sometimes may even interfere with the intended function of the thin film.

[0003] Various methods for producing sputter targets made of Si—Al are known: melting methods with slow ingot solidification, the casting of thin tiles (as described in DE 198 10 246 A1), or powder-metallurgical (P/M) methods such as hot pressing, hot isostatic pressing, or thermal spraying.

[0004] Particularly in the production of cylindrical tube cathodes, thermal spray methods, such as flame spraying and plasma spraying, have been found to be effective. A mixture of commercial elemental silicon powder and aluminum powder is used in these processes. A process of this kind is described in EP 0,586,809 B1, according to which the individual silicon particles are supposed to be at least partially coated with a layer of aluminum.

[0005] However, the following should be noted: commercial silicon and aluminum powders usually have different particle shapes. Whereas the particles of a silicon powder often have the angular structure of a grinding process, the particles of an aluminum powder always have a spherical to oblate spheroidal geometry. In all P/M processes in which two powders that differ in this respect must be mixed, there is the risk that powder segregation will occur during production of the product. Especially when the sputter target is produced by a thermal spray process in which the powder to be sprayed is fed by a feed mechanism of the spray gun, the result can be large deviations in the target composition. Furthermore, when there are large differences in melting point and vapor pressure, as in the case of silicon and aluminum, there is a large risk that one of the components will become enriched or depleted in the spray coating.

[0006] Measurements have shown that relative variations of up to 80% can occur in the Al content of a 3,700-mm-long, plasma-sprayed Si—Al target for a tubular cathode. Even when other P/M production methods are used, such as hot pressing or hot isostatic pressing, there is the risk of segregation during the processing of multicomponent powders with different sizes and morphologies of the components.

[0007] Naturally, variations of this magnitude in the Al content of a sputter target are undesirable, since they can cause serious deviations in the optical parameters of thin coatings applied with the use of the sputter target.

SUMMARY OF THE INVENTION

[0008] Therefore, an object of the present invention is to provide an Si—Al sputter target that shows only slight variation in Al content or, if at all possible, no variation at all, and to provide a process for producing sputter targets of this type.

[0009] In accordance with the invention, this goal is achieved by a sputter target that contains a larger fraction of silicon and a smaller fraction of aluminum and is produced from a powdered silicon-aluminum alloy by a forming and/or compacting process.

[0010] Silicon is a commonly used coating material that is intended to affect the optical properties of the coated substrate. Aluminum is a material that is widely used to reduce the brittleness of a sputter target that is composed mainly of silicon. In a number of special cases, other materials are also used, and the process of the invention may be similarly used with these materials.

[0011] The sputter target of the invention is thus characterized by the fact that it is not produced from two different types of powders, namely, typically an Si powder and an Al powder, but rather is produced from a single, uniform (although two-phase) Si—Al alloy powder. The Si—Al alloy powder to be used preferably has an Al content of 0.1 to 30 wt. % and especially 5-15 wt. %. However, alloys with other Al contents can be produced and used.

[0012] In particular, the sputter target is characterized by the fact that it is produced by first producing a melt that consists of silicon and aluminum, then using the melt to produce a powder, whose two-phase particles consist of the aluminum-silicon alloy, and, finally, applying the powder to a target support by a P/M production process to produce the sputter target.

[0013] An outstanding characteristic of this alloy powder is the fine precipitation distribution of the Al in the Si particles. Surprisingly, this extremely homogeneous distribution of the Al phase is maintained even when thermal spraying is used as the forming process.

[0014] Since the Al phase is present in the form of Al precipitations in the Si particle, it is protected during the thermal spray process by the Si particle that surrounds it and cannot vaporize. This circumstance probably explains why the Al content of the alloy powder is maintained with minimal variation even in the spray coating and in the target material. This occurs contrary to the theory advanced in EP 0,586,809, according to which the aluminum supposedly surrounds the silicon particle.

[0015] It was found to be especially advantageous for the particles of the alloy powder to be as close to spherical in shape as possible. Especially when a thermal spray process is used as the forming process, this leads to especially high densities and especially high application rates and thus to a very economical production process.

[0016] Since the production of the sputter target requires that a special powder be produced first, namely, the alloy powder, which is not commercially available, there is a small cost disadvantage relative to state-of-the-art sputter targets. However, the cost disadvantage of the alloy powder can be almost completely compensated by using a gas atomization process to produce the powder from a melt in the presence of air. Surprisingly, it turns out that the oxygen content after the melt has been gas atomized in the presence of air approximates the low gas contents of conventional powder mixtures. Therefore, no disadvantages due to gas atomization of the melt in the presence of air would be expected in the subsequent sputtering process.

[0017] The goal of the invention is achieved by another sputter target that is produced from spheroidal silicon particles that form a silicon matrix, in which the aluminum is finely and/or extremely finely distributed in the silicon particles. Whereas in a sputter target produced by a state-of-the-art process, particles of silicon and aluminum are arranged side by side, so there may be significant variation in the distribution of the aluminum in the silicon, in a sputter target produced in accordance with the invention, aluminum is incorporated in a silicon matrix formed from Si particles, which ensures that the aluminum is uniformly distributed in the sputter target.

[0018] The Al content of the sputter target is preferably 0.1 to 30 wt. % and especially 5-15 wt. %. However, sputter targets with other Al contents can be produced and used.

[0019] It was found to be especially advantageous for the aluminum to be incorporated in the silicon particles in the form of finely or extremely finely distributed aluminum spots. This ensures that the aluminum is in fact uniformly distributed in the sputter target material.

[0020] The invention also concerns a process for producing the sputter targets described above. The process should be inexpensive and must produce a sputter target that has a composition that is as uniform as possible. In particular, the aluminum content should not be subject to any spatial variation.

[0021] The goal of the invention with respect to a process for producing a sputter target of this type, which is composed essentially of silicon and aluminum and contains a larger fraction of silicon and a smaller fraction of aluminum, is achieved by producing the sputter target from a powdered silicon-aluminum alloy by a forming and/or compacting process.

[0022] The required silicon-aluminum alloy in powdered form is preferably obtained by first producing a melt composed of silicon and aluminum and then using the melt to produce a powder, whose two-phase particles consist of an alloy of the two materials. The powder is then applied to a target support by a P/M production process to form the sputter target.

[0023] It was found to be effective to produce a melt with an aluminum content of 0.1 to 30 wt. % and especially 5-15 wt.% and the remainder silicon.

[0024] The alloy powder composed of the two phase particles is preferably applied to the target support by a thermal spray process.

[0025] This process works especially well if the powder particles have a spheroidal shape.

[0026] A preferred process for producing the powder is gas atomization of the melt in the presence of air.

[0027] The invention will now be explained by an example. The difference from the state of the art is illustrated by two figures, which show a state-of-the-art sputter target and a sputter target of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 shows the structure of a target plasma sprayed with the alloy powder in accordance with the invention; and

[0029]FIG. 2 shows the structure of a target produced by a conventional method.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The production of the target of the invention will now be explained by an example. First, an alloy powder is produced:

Alloy Powder

[0031] To produce the alloy powder, an air-exposed melt composed of 90 wt.% Si and 10 wt. % Al (abbreviated SiA110) is atomized under an inert gas. The atomized SiA110 powder is then screened to a particle-size fraction of 45-150 μm for the purpose of plasma spray application. The resulting powder consists primarily of particles with a spheroidal shape.

[0032] The oxygen content of this powder and of a comparative powder are shown in Table 1. TABLE 1 Powder Mixture Composed of SiAl10 Alloy Powder 90 wt. % Si and 10 wt. % Al oxygen content 0.04 up to 0.55 [%]

[0033] The alloy powder is then formed into a sputter target.

Forming

[0034] If the SiA110 alloy powder is plasma sprayed under an argon stream in standard ambient atmosphere (APS—atmospheric plasma spraying), structures of the type shown in FIG. 1 are produced. For comparison, FIG. 2 shows the results obtained by a state-of-the-art process, i.e., the structure of a target plasma sprayed with an Si/Al powder mixture.

[0035] It is clear that the target produced by the process of the invention is more homogeneous.

[0036] The SiA110 powder can also be sprayed under “vacuum,” i.e., low-pressure plasma spraying (LPPS). The resulting oxygen content of the target material can be further reduced by this method, since the spraying atmosphere is kept free of ambient oxygen.

[0037] The process of hot isostatic pressing (HIP) is also suitable for forming the powder. In this case, the alloy powder is placed in an HIP container and compacted at temperatures of 600-1,100° C. and a pressure of 2,000 bars.

[0038] Table 2 gives material data of SiA110 targets produced either from an Si/Al mixed powder or from an SiA110 alloy powder in accordance with the invention.

[0039] Table 2 clearly shows that a constant value is always established, regardless of the application process, especially where the Al content is concerned. TABLE 2 Target produced with powder mixture containing 90 wt. % Target produced with Si and 10 wt. % Al powder SiAl10 alloy powder LLPS APS HIP LLPS APS HIP 0.07 0.57 0.56 oxygen 0.09 0.46 0.08 content [%] 2.20 2.18 2.31 density 2.18 2.16 2.32 [g/cm³] 5-30 0.5-40 70-200 Si particle 1-50 1-50 45-150 size [μm] 10-100   15-100 50-150 precipitate 1-10 1-10 1-10 size [μm] 9-18   9-18 10-12  Al content 10.0 9.8 9.9 [%]

[0040] The result is seen in FIG. 1. The alloy powder forms a structure in the sputter target that consists of individual spherical particles about 50 μM in diameter with smaller satellite spheres adjacent to them. Each spherical particle consists essentially of silicon, which appears light gray in the micrograph. Fine and extremely fine aluminum spots are uniformly distributed in the spherical particles and appear as small white spots with maximum diameters of 5 μm. Since each spherical silicon particle encloses such spots, the aluminum is uniformly distributed through the sputter target. The dark areas represent pores in the sputter target. They occupy a volume of ca. 10% of the sputter target.

[0041] The quite different structure of the target of the invention is also apparent from a comparison with the sample in FIG. 2, which shows a sputter target produced by conventional methods. Since this involves the application of a mixture of powders on the target support, large stratified areas of aluminum and silicon are formed. The silicon appears as light-gray areas, and the aluminum appears as white areas in the micrograph. Especially at the upper edge of the micrograph, we see an elongated white area ca. 120 μm wide and 20-30 μm high. The dark areas are pores. The coarse agglomeration of the aluminum prevents significant reduction of the brittleness of the sputter target.

[0042] A sputter target produced by the process of the invention with the use of a powder produced by gas atomization of an SiAl melt can be readily recognized by the fact that finely and extremely finely distributed aluminum is incorporated in the silicon, which is present in the form of nearly spherical particles, which are much larger than the aluminum spots.

[0043] Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 

We claim:
 1. A sputter target composed essentially of silicon and aluminum and contains a larger fraction of silicon and a smaller fraction of aluminum, the sputter target being formed from a powdered silicon-aluminum alloy by at least one of a forming process and a compacting process.
 2. A sputter target in accordance with claim 1, wherein a melt consisting of silicon and aluminum is first produced, the melt is used to produce a powder with two-phase particles that consist of an alloy of the silicon and aluminum, and the powder is applied to a target support by a powder-metallurgical production process to produce the sputter target.
 3. A sputter target in accordance with claim 2, the alloy powder that consists of the two phase particles is applied to the target support by a thermal spray process.
 4. A sputter target in accordance with claim 1, wherein the powder particles have a spheroidal shape.
 5. A sputter target in accordance with claim 2, wherein the powder is produced by gas atomization of a melt in the presence of air.
 6. A sputter target in accordance with claim 2, wherein the silicon-aluminum alloy powder and the sputter target produced from it have an aluminum content of 0.1 to 30 wt. %.
 7. A sputter target in accordance with claim 6, the silicon-aluminum alloy powder has an aluminum content of 5-15 wt. %.
 8. A sputter target composed essentially of silicon and aluminum, the sputter target consisting of spheroidal silicon particles that form a silicon matrix in which the aluminum is finely or extremely finely distributed in the silicon particles.
 9. A sputter target in accordance with claim 8, wherein the silicon-aluminum alloy powder and the sputter target produced from it have an aluminum content of 0.1 to 30 wt. %.
 10. A sputter target in accordance with claim 9, wherein the silicon-aluminum alloy powder has an aluminum content of 5-15 wt. %.
 11. A sputter target in accordance with claim 8, wherein the aluminum is incorporated in the silicon particles as finely or extremely finely distributed aluminum spots.
 12. A process for producing a sputter target that is composed essentially of silicon and aluminum and contains a larger fraction of silicon and a smaller fraction of aluminum, comprising the step of forming the sputter target from a powdered silicon-aluminum alloy by at least one of a forming process and a compacting process.
 13. A process in accordance with claim 12, including first producing a melt consisting of silicon and aluminum, producing a powder from the melt, which powder has two-phase particles consisting of an alloy of the silicon and aluminum, and applying the powder to a target support by a powder-metallurgical production process to form the sputter target.
 14. A process in accordance with claim 13, including producing the melt with 0.1 to 30 wt. % aluminum and a remainder silicon.
 15. A process in accordance with claim 14, including producing the melt with 5-15 wt. % aluminum and the remainder silicon.
 16. A process in accordance with any of claim 13, including applying the alloy powder that consists of the two-phase particles to the target support by thermal spraying.
 17. A process in accordance with claim 12, wherein the powder particles have a spheroidal shape.
 18. A process in accordance with claim 12, including producing the powder by gas atomization of a melt in the presence of air. 